1. Field of the Invention
The present invention relates to a toner useful for electrophotography, and to an image forming method using the toner. More particularly, the present invention relates to a toner useful for an electrophotographic image forming method in which toner images are transferred from an image bearing member to a receiving material via an intermediate transfer member.
2. Discussion of the Background
Image forming apparatus such as copiers, printers and facsimile machines using the following electrophotographic image forming method are well known:
(1) an electrostatic latent image is formed on an image bearing member such as a photoconductor; PA1 (2) the latent image is developed with a toner to form a toner image on the image bearing member; PA1 (3) the toner image is transferred to an intermediate transfer member; PA1 (4) the toner image transferred on the intermediate transfer member is then re-transferred on a receiving material such as a paper sheet; and PA1 (5) the toner image on the receiving material is fixed, for example, upon application of heat, to form a fixed toner image. PA1 (1) position differences between color images transferred on a receiving material can be improved; PA1 (2) even when various receiving materials having different characteristics are used, toner images are clearly transferred on the receiving materials. PA1 (1) one or more fatty acid metal salts such as zinc stearate, and fluorine compounds are included in a top layer of the intermediate transfer member; and PA1 (2) a coating mechanism is provided for coating one or more fatty acid metal salts, such as zinc stearate, and fluorine compounds, on the intermediate transfer member. PA1 (1) a filming problem in which a film of the fatty acid metal salt or the acrylic polymer is formed on the intermediate transfer member; and PA1 (2) a problem in which the fatty acid metal salt or the acrylic polymer in the toner accumulates in various parts of the developing unit of the image forming apparatus when the toner is used for a long time, resulting in deterioration of fluidity and charging properties of the toner, and thereby image qualities deteriorate.
This image forming method is particularly useful for forming a color image. For example, a yellow (Y) toner image, a magenta (M) toner image, a cyan (C) toner image and a black (Bk) toner image, which are formed on a respective image bearing member, are transferred on an intermediate transfer member one by one to form a color toner image thereon. The color toner image is transferred on a receiving material at a time and then fixed. Thus, a color image can be prepared.
The advantages of image forming apparatus using such an intermediate transfer member over image forming apparatus in which toner images formed on an image bearing member are directly transferred on a receiving material are as follows:
Therefore, the image forming apparatus having an intermediate transfer member are preferably used as color image forming apparatus.
In general, a cleaning mechanism, such as a blade cleaning device, a fur blush cleaning device, a web cleaning device, or a magnetic blush cleaning device, is generally provided in such image forming apparatus having an intermediate transfer member to remove the toner, which remains on the intermediate transfer member without being transferred. However, these cleaning devices cannot perfectly remove the remaining toner, and the toner still remaining on the intermediate transfer member gradually accumulates thereon, resulting in formation of a toner film on the intermediate transfer member. When the toner film is formed on the intermediate transfermember, the transfer ability of the intermediate transfer member gradually deteriorates. Therefore uneven toner transferring occurs or a white spot problem in that white spots are formed in the transferred toner images tends to occur, resulting in deterioration of the image qualities of the resultant toner images (this problem is hereinafter referred to as a toner filming problem).
In image forming apparatus using a belt-shaped intermediate transfer member, both of first image transferring in which toner images are transferred from an image bearing member to the belt-shaped intermediate transfer member, and second image transferring in which the toner images are transferred from the transfer belt to a receiving material are performed using a roller. Namely, the toner images on the image bearing member are transferred on the transfer belt while the belt is pressed by a roller toward the image bearing member. Similarly, the toner images on the transfer belt is transferred on a receiving material while the belt is pressed by a roller toward the receiving material. By using a roller in toner transferring, adhesion of toner particles to the image bearing member, the intermediate transfer member and the receiving material is increased. In addition, adhesion between the toner particles is also increased, resulting in formation of toner aggregates. When toner aggregates are formed, a high pressure is locally applied to the aggregates by a roller, resulting in formation of omission in toner images as if the toner images are eaten by worms (hereinafter these images are referred to be worm-eaten images). In particular, the worm-eaten images are observed in line images.
In order to avoid the toner filming problem and the worm-eaten images, the following measures are taken:
However, even when these measures are taken for an intermediate transfer member, uneven transferring or white spots are generated if the intermediate transfer member is used for a long time. Therefore the intermediate transfer member has a short life. In addition, when a coating mechanism is provided in an image forming apparatus, the image forming apparatus becomes complex and large-sized, resulting in increase of manufacturing costs.
Techniques are also well known in which a fatty acid metal salt such as zinc stearate, or a particulate acrylic polymer is included in a toner in order to impart good cleaning properties to a toner. However, such a toner is used for an image forming apparatus having an intermediate transfer member, the following problems tend to occur:
Japanese Laid-Open Patent Publication No. 8-314231 discloses an image forming method in which an intermediate transfer member is used without cleaning. Namely, the toner remaining on the intermediate transfer member is removed by a transfer roller and the like to which a voltage having a polarity opposite to that of the toner is applied to catch the remaining toner. However, the toner cannot be sufficiently removed from the intermediate transfer member by such a method. In addition, this method has a drawback in that a new cleaner must be provided in the image forming apparatus to remove the toner adhered on the transfer roller.
Because of these reasons, a need exists for an image forming method and apparatus using an intermediate transfer member which can produce images having good image qualities for a long time.